Thermoplastic materials are used to manufacture items that range from food packages to automotive body panels. In many cases, particularly in cases where superior product performance or appearance is required, multi-layer or multi-material constructions are utilized. Such applications could include: painted plastic parts, plastic packages with barrier coatings, clear coated plastic parts, co-extruded or co-molded items, multi-shot molded parts, and in-mold paint film applications.
In each of the cases listed above, the presence of the multi-material structure creates problems with respect to the mechanical “recyclability” the materials. The other material layers will often act as incompatible contamination, and have a generally negative impact on the recyclability of the primary thermoplastic material. For example, most of the paints or coatings used to paint plastic parts are thermosetting in nature and act as solid particulate inclusions in the recycled matrix material. Ideally, recovered materials are fully separated or segregated prior to recycling. This provides the highest value recycle stream(s).
Methods for removing coatings from coated plastic parts have been developed, but are not always cost effective. Similarly, separating multi-material or multi-layer structures may or may not be technically feasible depending on their specific construction. However, the cost of segregating multi-material structures can often be more than economics justify. In other cases, such as in the case of a complex automotive carpet structure, separation of the individual plastic constituents that make up the structure is not technically feasible. In such cases, the multi-material formulation is most easily reused as a commingled stream. In some cases, additional materials or compatibilizing agents are used to enhance the physical properties and/or processability of the commingled feed. As an alternative, these complex material constructions can sometimes be effectively recycled if the contaminating layers can be reduced in particle size and effectively mixed into the continuous thermoplastic matrix material.
Studies have shown that the “degree of mixing” is an extremely important variable when reprocessing commingled plastic materials or a contaminated plastic formulation. The same exact material composition can exhibit very different physical properties if it is melt processed at different temperatures or with a different mixing history (or degree of mixing). Mixing during compounding and/or reprocessing are both important. The specific nature of the materials involved will determine whether distributive or dispersive mixing (or both) is most important.
An example of a multi-layer plastic item is a painted thermoplastic automotive body panel, such as a thermoplastic polyolefin bumper coated with a primer/paint system. In-plant (post industrial) recycling is important for painted parts that are rejected for quality reasons, however post consumer recycling of painted plastic automobile body panels becomes more important by the day. If a painted or coated thermoplastic product is simply granulated and extruded (or injection molded) into a new, second generation plastic item, the end product will likely exhibit inferior mechanical properties and surface finish. Most paints or coatings used in such applications are thermosetting in nature, and do not re-melt when the substrate thermoplastic material is reprocessed. The un-melted paint flakes can act as contamination in the recycled plastic matrix, resulting in mechanical property and surface finish quality problems.
There are a variety of ways to deal with the problems associated with reprocessing of painted or coated reground plastic. It is possible to remove the coatings from the plastic granules by methods such as chemical attack, differential thermal expansion, abrasion, etc. However, all of these techniques involve additional processes, handling, equipment and significant cost. Other methods of paint removal involve chemical degradation of the paint film and removal of the volatile degradation products. Melt filtration has also been used to remove coatings. Many other paint removal methods have been developed using other concepts such as chemical stripping, autoclave treatment, differential thermal expansion and pulverization.
One possible alternative to the decontamination techniques described above is to reuse the contaminated plastic without removing the paint coating or other non-melting contaminate. Eliminating the pre-processing paint removal steps would reduce the overall reprocessing costs during recycling, but unfortunately these cost reductions are at the expense of material performance or quality. However, there are a number of secondary recycling applications where the physical properties of the contaminated material may still be adequate for the application. One factor that is known to be important for contaminated plastics is the physical “size” and “shape” of the contaminating particles. Generally, a smaller particle, with more uniform particle size distribution, results in a more homogeneous material. Particle size reduction for solid contaminates can be achieved through granulation, or through some type of melt mixing action. Molded plastic parts normally have a wall thickness in the 1.0 mm to 3.0 mm range. A typical granulator used for granulating painted plastic parts uses a 10 mm diameter discharge screen. The granules (and paint coating) discharged from the granulator can have length and width dimensions as large as this screen hole diameter. Therefore, size of the paint flakes (i.e. their width and length) associated with granulated plastics are typically greater than the thickness of a typical molded plastic part. It is expected that the surface appearance of molded parts produced from reground painted plastics would be improved if the contaminating flakes have a smaller physical size. Smaller size contamination will also cause fewer problems if the contaminated melt flows through thin wall sections or small orifices (such as a hot runner gate). The physical size of the paint flake contamination can be reduced by regrinding the painted parts with a granulator having a smaller discharge screen diameter. However, this approach presents problems with both granulator throughput rates, and possible material handling difficulties (fines and powder).